A small scanning tunnelling microscope with large scan range for biological studies.

We have developed a scanning tunnelling microscope specially designed for biological application presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5x20 microm with a vertical range of 20 microm and the total size of the system does not exceed 10x40 mm. The piezo-sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d13 of -1.71 A/V at low voltages (under a few volts) going up to -2 A/V for higher voltages. Large-scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.